While water covers approximately two-thirds of the planet, most of this water is too salty to be of any use. As the demand for water continues to increase along with a growing world population, the treatment of wastewater has become ever more important.
Conventional biological wastewater treatment processes typically utilize an activated sludge configuration and incorporate at least two tanks: an activated sludge reactor and a secondary clarifier. This activated sludge process is used to treat organic carbon (e.g. measured as BOD, or biochemical oxygen demand) and suspended solids. The secondary clarifier is required in order to remove sludge and suspended solids.
To achieve advanced treatment of nutrients such as nitrogen and phosphorus, additional treatment phases must be incorporated. A conventional biological nutrient removal (BNR) plant may include an additional two to three tanks. Although this BNR configuration can provide advanced secondary wastewater treatment with nutrient removal, their large land footprint and high cost often prohibit their installation, especially in decentralized applications where sewer mains are not already established.
For larger wastewater treatment plants, energy costs are a major contributor to operation costs and can amount to many hundreds of thousands of dollars per year. For example, an average secondary activated sludge treatment plant uses about 65% of its energy consumption for aeration.
Sequencing Batch Reactor (SBR) technology improves upon other activated sludge biological wastewater treatment processes in terms of space and cost because all phases of the treatment process (such as anoxic treatment, aerobic treatment and settling) can be conducted in one tank. Secondary clarifiers are not required.
The SBR treatment sequence consists of a repeating cycle of treatment phases: fill, anoxic (optional), aerobic, settle, and decant.
SBR technology has reduced the land footprint and cost associated with conventional activated sludge processes. Nevertheless, one shortcoming of biological treatment processes that is inherent to both activated sludge and SBR processes at large is the lack of advanced process control. Conventional SBR process control typically includes presetting the length of the treatment phases (such as aerobic and anoxic phases) based on the operator's estimate of wastewater loading. This type of SBR operation is called fixed-time control. Because wastewater loading may be variable throughout the day, week or season, fixed-time control tends to either over-aerate or under-aerate the wastewater. Over-aeration leads to excessive energy consumption as well as compromised sludge performance. Under-aerated cycles do not fully treat the wastewater.
The use of real-time reactor monitoring, diagnostics and data collection considered ‘advanced process control’ is uncommon. Furthermore, many SBR systems do not use dissolved oxygen (DO) control for their processes, resulting in very limited control over anoxic and aerobic biological processes.
Although research and development on control of SBR aerobic phase treatment length using DO measurements in real-time (“real-time control”) has been studied in the lab and in pilot-scale reactors, this research has not been widely commercialized. For example, a study by Battistoni et al., (Ind. Eng. Chem. Res., 2003, 4:509-515), at the University of Ancona in Italy used a patented (Battistoni, P., Italian Patent No. NR99A000018, 1999) process controller which interprets ORP and DO signals to control the aeration in a small extended aeration wastewater treatment plant. The paper claims that the controller uses DO detection of the ammonia elbow to control the aeration cycle length. This technology differs from the present technology in that it does not contain automatic DO calibration and real-time airflow control. Without automatic DO sensor calibration, the real-time DO measurements may not be accurate thus limiting the effectiveness of the overall process to only those situations in which the DO sensors are manually calibrated. Without real-time airflow control, it is questionable if the DO of the system can be kept within the specific range (as in the present invention) known to consistently detect the ammonia elbow in the DO profile and to ensure complete nitrification and treatment. Accordingly, the results showed that complete nitrification (ammonia removal) was inconsistent.
Other companies, for example Ondeo Services located in Australia, have commercialized technology which applies the use of other sensor parameters. Ondeo Services OGAR® (Optimized manaGement of Aeration by Redox) process control technology uses oxidation reduction potential (ORP) to control nitrification (conversion of ammonia to nitrate) and to control the aeration length.